The present invention relates to an optical type information recording medium and, more particularly, to an optical-disk for recording and/or reconstruction of information signals using an energy beam such as laser.
Recently, optical-disks have been developed as one type of recording media which are capable of recording information at high density and big capacity. In a recording/reconstruction apparatus which uses an optical-disk of this type, tracking grooves about 0.6 .mu.m in width and 1.5 to 2 .mu.m in pitch are formed in the optical-disk in advance, and a focused laser beam having a spot diameter of about 1 .mu.m is radiated on the groove to record information signals. The tracking control is performed through detection of the changes in the levels of reflected light signals and transmitted light signals which correspond to the positional relationship between the tracking grooves and the beam spot of the laser beam on the optical disk. The recording/reconstruction apparatus includes a differentiator and a single axis position sensor which receives the light signals by dividing them into halves to generate first and second electric signals. When the center of the laser beam spot coincides with the center of the tracking groove, the single axis position sensor generates first and second electric signals which do not have any level difference. When the center of the beam spot deviates from the center of the tracking groove in the radial direction of the optical-disk, the single axis position sensor generates first and second electric signals which have a level difference which depends on the direction and magnitude of this deviation. By differentiating this level difference, the deviation (tracking error) may be detected. By correcting this tracking error, the laser beam spot is returned to central position and tracking control may be performed with high precision.
In order to reconstruct the information recorded in this manner, the change in the intensity of the reflected light, which corresponds to the presence or absence of a pit which is formed in the tracking groove of the optical-disk, is detected by an adding amplifier which adds the first and second electric signals which are generated by the single axis position sensor.
The shape of the tracking groove formed in the optical-disk as described above is preferably such that the influence of the presence of the tracking groove on the addition result obtained from the single axis position sensor may be reduced to the minimum, and yet the percentage modulation of the differential output may be increased: which output corresponds to the deviation of the center of the laser beam spot from the center of the tracking groove, that is, the tracking error. As an example of the shape of the tracking groove, Japanese Patent Disclosures (KOKAI) No. 55,448/80 and No. 55,449/80 propose a V-shaped groove which has an inclination angle of about 80 to 85 degrees. IEEE SPECTRUM AUGUST, 1979, pp. 26 to 33 proposes a rectangular groove according to which the phase difference between the signals obtained in correspondence with the interior and exterior of the tracking groove equals .lambda..multidot.(m/8), where m is an odd integer and .lambda. is the wavelength of the laser beam used.
However, with an optical-disk having the conventional tracking groove as described above, the allowable error of the depth of the tracking groove becomes extremely small. For this reason, during the actual manufacture of an optical-disk, it is extremely difficult to form a tracking groove on the optical-disk to satisfy the requirement of the allowable error of the depth of the tracking groove.